In recent years, as magnetic recording devices, such as magnetic disk drives, flexible disk drives and magnetic tape drives, have immensely expanded their ranges of utility and gained in significance, efforts have been directed toward enabling the magnetic recording media used in these drives to be prominently improved in recording density. Particularly, the increase in surface recording density has been further growing in ardency since the introduction of the Magnet Resistive (MR) head and the Partial Response Maximum Likelihood (PRML) technique. Owing to the further introduction of the Giant-Magnet Resistive (GMR) head and the Tunneling Magneto Resistive (TMR) head in recent years, the increase is continuing at a pace of about 100% per year. These magnetic recording media are being urged to attain a still higher recording density in future and their magnetic recording layers to accomplish addition to coercive force, Signal to Noise Ratio (SNR) and resolution. Recent years have been witnessing efforts that are being continued with the object of enhancing the linear recording density and adding to the surface recording density by increasing the track density as well.
In the latest magnetic recording devices, the track density has reached 110 kTPI. As the track density is further increased, it tends to entail such problems as causing interference between the parts of data magnetically recorded in adjacent tracks and inducing the magnetization transition region in the borderline region to constitute a noise source and impair the SNR. This fact hinders the enhancement of the recording density because it immediately results in lowering the bit error rate.
For the sake of increasing the surface recording density, it is necessary that the individual recording bits on the magnetic recording medium be formed in as minute a size as possible and enabled to secure as large saturated magnetization and magnetic film thickness as permissible. As the recording bits further decrease in size, however, they tend to entail such problems as lessening the minimum volume of magnetization per bit and inducing extinction of recorded data through the magnetization reversal caused by thermal fluctuation.
Further, since the track pitch grows small, the magnetic recording device necessitates a tracking servo mechanism of extremely high accuracy and, at the same time, generally needs adoption of the method of executing the recording in a large width and executing the reproducing in a smaller width than during the recording with a view to eliminating the influence from the adjacent tracks to the fullest possible extent. Notwithstanding that this method is capable of suppressing the influence between the adjacent tracks to a minimum, it entails such problems as rendering sufficient acquisition of the output of reproduction difficult and consequently incurring difficulty in securing a sufficient SNR.
As one means to cope with the problem of thermal fluctuation and accomplish acquisition of due SNR or a sufficient output, an attempt to enhance the track density by forming irregularities along the tracks on the surface of the recording medium and consequently physically separating mutually the adjacent tracks is now under way. This technique will be referred to as a “discrete track method” and the magnetic recording medium that is produced by this technique will be referred to as a “discrete track medium” herein below.
As one example of the discrete track medium, a magnetic recording medium that is formed on a nonmagnetic substrate bestowed on the surface thereof with irregular patterns and enabled to acquire physically separated magnetic recording track and servo signal pattern has been known (refer, for example, to JP-A 2004-164692).
This magnetic recording medium has a ferromagnetic layer formed on the surface of a substrate possessing a plurality of irregularities on the surface thereof via a soft magnetic layer and has a protecting film formed on the surface of the ferromagnetic layer. This magnetic recording medium has formed in the convexed regions thereof magnetic recording regions magnetically divided from the environments.
According to this magnetic recording medium, it is held that a high-density magnetic recording medium issuing no great noise can be formed because the fact that the occurrence of magnetic walls in a soft magnetic layer can be suppressed results in preventing the influence of thermal fluctuation from readily appearing and allowing extinction of interference between the adjacent signals.
The discrete track method is known in two kinds, i.e. a method which forms a track subsequent to forming a magnetic recording medium consisting of a number of stacked thin films and a method which forms a thin-film magnetic recording medium either directly on the surface of a substrate or subsequent to forming irregular patterns on a thin-film layer ready for the formation of a track (refer, for example, to JP-A 2004-178793 and JP-A 2004-178794). The former method, often called a magnetic layer processing type, is at a disadvantage in suffering the medium to be readily contaminated during the course of production and greatly complicating the process of production as well because it requires the physical processing of surfaces to be carried out subsequent to the formation of the medium. The latter method, often called an emboss processing type, though not inducing ready contamination during the course of production, is at a disadvantage in disabling stabilization of the posture and the height of floatation of the recording and reproducing head adapted to execute recording and reproducing while floating on the medium because the irregular shape formed on the substrate is fated to continue existence on the film to be formed.
The emboss processing-type method of production enables no easy realization of a flat surface because the irregular shape formed on the substrate is overlaid with the magnetic layer and the protecting layer and is consequently suffered to continue the existence thereof to the surface to be completed.
On the other hand, the discrete track-type recording medium by the magnetic layer processing-type method adopts a procedure of forming the magnetic layer used for recording on the surface of the substrate and subsequently forming a magnetic pattern and, therefore, acquires a structure that results from executing pattern formation by the imprinting method utilized as for semiconductors, subsequently dry-etching the part fated to form a nonmagnetic part, thereafter embedding SiO2 or a carbon-based nonmagnetic material, subjecting the resultant surface to a planarizing treatment, further coating the surface with a protecting film layer, and forming a lubricating layer thereon. This magnetic etching-type discrete track medium complicates the process of production and not only forms a cause for contamination but also fails to realize a flat surface.
Generally, the magnetic recording medium of such a structure as this enables enlarging the output and input signals through the head and heightening the recording density as well because the distance from the head to the magnetic layer decreases in accordance as the protecting film layer becomes thin. The pit density in the track is decided by the height of floatation of the head running on the surface of the protecting film layer of an irregular shape. How the floatation of the head is stably retained, therefore, constitutes an important task for the sake of accomplishing a high recording density. It is therefore required that the irregular pattern is capable of allowing the floatation of the head to be stably retained, enabling the head to approximate as closely to the magnetic layer as possible, and moreover preventing mutual interference of signals on the adjacent tracks.
However, a technique for producing a discrete track medium that entails scarcely the risk of causing contamination during the course of production and enables formation of a flat surface has not been proposed to date.
This invention is directed, in the magnetic recording device confronting technical difficulty in consequence of the increase in the track density, toward immensely increasing the track density and consequently increasing the surface recording density while ensuring acquisition of higher recording and reproducing properties than ever. Particularly in the discrete track-type magnetic recording medium adapted to execute formation of concaves and convexes subsequent to having a magnetic layer formed on a substrate, this invention is directed toward providing a method for production that exceptionally simplifies the process of production as compared with the conventional process of the magnetic layer processing-type by depriving this conventional process of a step for removing the magnetic layer and sparingly entails the risk of causing contaminants and toward a discrete track-type magnetic recording medium that excels in the head-floating property and proves to be useful.